Manacher Longest Palindromic Substring in PHP

<?php
declare(strict_types=1);
// Manacher Longest Palindromic Substring in PHP
//
// PURPOSE
// Manacher reuses mirrored palindrome radii to find a longest contiguous palindrome in
// linear time.
//
// METHOD
// Compute odd and even center radii in separate passes. Reuse the mirror radius within the
// current rightmost palindrome, then expand only beyond its known boundary.
//
// COMPLEXITY
// O(n) time and O(n) auxiliary space.
//
// CONTRACT
// Integer tokens. Returns longest palindrome LENGTH; empty input returns zero.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// PHP 8.1+ on a 64-bit runtime: packed arrays, strict scalar types, read-only inputs by
// value using copy-on-write, references only for outputs, and checked 32-bit arithmetic.
// Invalid input and overflow throw exceptions. PHP arrays have higher overhead than native
// packed buffers in C/Java.
//
// EXAMPLE
// Expected return: 4.
// Run: php example.php
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

function alg_require(bool $valid): void { if (!$valid) throw new InvalidArgumentException("invalid algorithm input"); }
function alg_checked(int|float $v): int { if ($v < -2147483648 || $v > 2147483647 || floor($v) != $v) throw new OverflowException("32-bit result overflow"); return (int)$v; }
function alg_add(int $a,int $b): int { return alg_checked($a+$b); }
function alg_sub(int $a,int $b): int { return alg_checked($a-$b); }
function alg_mul(int $a,int $b): int { return alg_checked($a*$b); }
function alg_size(int $n): int { alg_require($n>=0 && $n<=4000000); return $n; }

function manacher(array $a): int {
    alg_require(array_is_list($a));
    $n = count($a);
    alg_require((($n >= 0) && ($n <= 1000000)));
    alg_require((count($a) == $n));
    $radius = array_fill(0, alg_size(alg_add($n, 1)), 0);
    $parity = 0;
    $left = 0;
    $right = alg_sub(0, 1);
    $i = 0;
    $k = 0;
    $best = 0;
    while (($parity < 2)) {
        $left = 0;
        $right = alg_sub(0, 1);
        $i = 0;
        while (($i < $n)) {
            $k = alg_sub(1, $parity);
            if (($i <= $right)) {
                $k = min($radius[alg_add(alg_sub(alg_add($left, $right), $i), $parity)], alg_add(alg_sub($right, $i), 1));
            }
            while (((alg_sub(alg_sub($i, $k), $parity) >= 0) && (alg_add($i, $k) < $n) && ($a[alg_sub(alg_sub($i, $k), $parity)] == $a[alg_add($i, $k)]))) {
                $k = alg_add($k, 1);
            }
            $radius[$i] = $k;
            $best = max($best, alg_add(alg_sub(alg_mul(2, $k), 1), $parity));
            if ((alg_sub(alg_add($i, $k), 1) > $right)) {
                $left = alg_add(alg_sub(alg_sub($i, $k), $parity), 1);
                $right = alg_sub(alg_add($i, $k), 1);
            }
            $i = alg_add($i, 1);
        }
        $parity = alg_add($parity, 1);
    }
    return $best;
}

$a = [1, 2, 2, 1];
$result = manacher($a);
echo $result, PHP_EOL;